Crude oil is a complex mixture. Standard analytical techniques used to fractionate this mixture show that crude oil is composed of a variety of hydrocarbon chemistries. Paraffinic and aromatic components may contribute to the viscosity, “heaviness”, of the crude oil (Hyne, N.J. 2001, “Non-technical guide to petroleum geology, exploration, drilling, and production”, 2nd edition, Pen Well Corp., Tulsa, Okla., USA). Heavy oil is more difficult to recover than light oil. Improved oil recovery depends on the improved flow of the oil through the porous reservoir media to the production well. Flow improvement can be achieved in a number of ways. Administering a second fluid to the reservoir, such as water, to push the oil through the formation has been utilized. Injection of water, often called waterflooding, is commonly used to improve oil recovery in the middle to late stages of the life of an oilfield (Hyne, N.J. 2001, “Non-technical guide to petroleum geology, exploration, drilling, and production”, 2nd edition, Pen Well Corp., Tulsa, Okla., USA). Techniques to reduce oil viscosity have also been used in addition to, or independently, of water flooding to improve oil flow. Reduction of oil viscosity may be achieved by physical or chemical means. Increasing the oil temperature via techniques such as fire flooding or steam injection will reduce viscosity, improving flow, (Hyne, N.J. 2001, supra), but in most situations, the vast amount of heating required to mobilize the reservoir will be too costly. In addition, in regions where reservoirs exist near permanent frozen surface strata, e.g. permafrost, such heating can result in the undesirable melting and collapse of the surface strata. Viscosity reduction has also been achieved through in situ biological modification of oil chemistry. This process is dependent on the addition of an inoculum of industrially produced aerobic (oxygen dependent) microbes that degrade long chain paraffins in the oil (for example, U.S. Pat. No. 6,905,870 B2). However, pumping air into the formation is costly and oxygen is highly corrosive to most equipment used in the oil field. Also, oil dominated by aromatic compounds cannot be modified by the described biological process because the microbial modification is specific to straight chain compounds. Yet, pi-bond interactions in aromatic compounds will contribute to oil viscosity (Sygula, A., J. Am. Chem. Soc., 129, 3842-3843, 2007).
Microbial transformation by reduction of organic compounds has been observed in various environments. Cholesterol is reduced to coprostanol in the gut by microbes, including the genus Eubacterium (Eyssen, H J et al Eur. J. Biochem. 36, 411-421, 1973). Humic acids in soils are reported to act as electron acceptors for anaerobic microbial respiration (Lovley D R et al., Nature, 382, 445-448,1996). Microbial reduction of azo compounds has been used to remove color from aqueous wastes (dos Santos, A B et al Enzyme and Microbial Technology. 39: 38-46, 2006). Some members of the genus Acetobacterium are reported to reduce acrylic groups in cinnamate derivatives (Foroughi, F et al., Enzyme and Microbial Technology. 39, 1066-1071, 2006). Microbial reduction of halogenated compounds under anaerobic conditions also serves as an important application in bioremediation for environmental pollutants (e.g. Tandol, V Environ. Sci. & Technol., 28, 973-979,1994). As reflected in these reports, microbial transformation by reduction of polyaromatic compounds is not known to the art of biochemical transformation by natural microbial populations.
Transformation of polyaromatic compounds to alicyclic compounds has been described from pure culture experiments in the laboratory (Safinowski, M and Meckenstock R U, Environ. Microbiol., 8, 347-352, 2006). For example, 5, 6, 7, 8-tetrahydro-2-naphthoic acid is reported in cell extracts from anaerobic sulfate reducers during the degradation of aromatic derivatives (Annweiler, E Appl. Environ. Microbiol., 67, 5077-5083, 2001), but evidence for such transformations is rarely found in the environment (Safinowski, M et al Environ. Sci. Technol. 40, 4165-4173, 2006). The process described herein is designed to encourage the development of natural microbial populations that will promote reduction of aromatic compounds associated with oil.
Previous microbial approaches to aromatic modification have generally used oxidative microbial metabolism. In an environment with excess organic matter, such as an oil reservoir, oxidizing agents such as oxygen or nitrate will be scavenged by microbes that are not active in aromatic modification. This lowers the efficiency of oxidant usage for aromatic modification. In contrast the reductants described in this case are not scavenged for organic matter oxidation, which leads to a more efficient microbial aromatic modification process. In addition, oxidative microbial metabolism often leads to hydrocarbon degradation, which can result in decreased oil recovery (Grishchenkov, V G et al Appl. Biochemi. Microbiol. (Translation of Prikladnaya Biokhimiya i Mikrobiologiya, 38,125-128, 2002). Reductive transformation does not cause hydrocarbon degradation.
Biochemical reduction of aromatic compounds in oil requires a reductant with a relatively low equilibrium redox potential, EH. Therefore, the reductants used to drive aromatic reduction must be able to supply electrons at a low redox potential. CO and H2 are examples of such reductants that can also be easily generated from precursors available at the oil well site such as natural gas and air. In addition, CO is also available to only a restricted group of anaerobic microbes, and therefore encourages the development of desirable populations. However, other materials that are good sources of low potential electrons, for example, formic acid, formate, 2-methoxy ethanol or sodium lactate or mandellic acid, or trimethylamine or ethylene glycol, may also be used to drive biochemical reduction of aromatic compounds.
Thus there is a need for developing means to reductively transform environmental chemicals particularly aromatic constituents of an oil reservoir to assist crude oil recovery.